Electromechanically active materials, i.e., materials developing mechanical stress in response to the application of an external electric field are of a great importance for a wide range of applications ranging from electromechanical transducers to micro-pumps. There are two major classes of materials currently in use: piezoelectric and electrostrictive. Classical piezoelectric and electrostrictive materials have electromechanical response well below 1 nm/V, typically 0.2-0.3 nm/V. This implies that generation of a 1 μm displacement requires application of a few thousand volts of external bias. No noticeable improvement in performance has been achieved during the last three decades. Although both piezoelectric and electrostrictive materials may develop stress of hundreds of MPa, small displacements restrict their field of applications. There are materials that may generate large displacements under application of electric fields (10-100 nm/V), i.e. the so-called porous electrets. However, the stress in these materials does not exceed a few MPa, making them unusable for most of applications requiring generation of a force above a few mN. From this standpoint, a material that can generate large displacements and large force may be of a considerable practical importance.
Microscopic origin of inelastic effects in Ce0.8Gd0.2O1.9 
Thin films of Ce0.8Gd0.2O1.9, one of the most important ionic conductors, exhibit a number of elastic anomalies, i.e. both spontaneous changes in lattice parameter as well as inelastic effects. The most striking of these is the ability to exhibit two different elastic moduli depending on time scale. This phenomenon, which has been called the chemical strain effect, can cause an absolute change in volume of ˜0.2% even if the external stress is homogenous. As such, the internal reorganization of point defects has been cited as a probable cause of the inelastic behavior (chemical strain) rather than the more commonly observed stress gradient-induced diffusion. Recently, the local bonding in Ce0.8Gd0.2O1.9 and in CeO2-x (x=0-0.05) was studied by extended X-ray absorption fine structure (EXAFS) spectroscopy. This work revealed that in Ce0.8Gd0.2O1.9, the cation-O bond is shorter than would be expected on the basis of the X-ray diffraction measurements of the average fluorite structure. Analyses of the 1st and 2nd coordination shells of Ce and Gd provided evidence that (1) the average distance from a Ce ion to an oxygen vacancy is larger than the mean Ce—O bond length and that (2) supported the theoretical finding that oxygen vacancies induced by Gd-doping prefer coordination with Ce ions rather than with Gd ions. Furthermore, it was found that in the presence of compressive strain of 0.3±0.1%, the average Ce—O bond length is decreased by 1±0.5%. The rearrangement of the Ce—O bond under strain was cited as a probable cause of the elastic anomalies in Ce0.8Gd0.2O1.9. The scenario for the chemical strain effect that was deduced from these studies is as follows. At room temperature, the cations and the anions shift with respect to each other so that Ce ions are observed to move away from the oxygen vacancies, locally distorting the symmetry. The fact that self-supported films of Ce0.8Gd0.2O1.9 spontaneously buckle at room temperature suggests that this shift of the Ce ions results in an initial volume increase. Heating decreases the repulsion between the Ce4+ ions and the oxygen vacancies, thereby restoring the more symmetrical environment and leading to film flattening. The activation energy deduced from the film flattening time is comparable to that measured for the self-supported Ce0.8Gd0.2O1.9 films. This suggests that the microscopic processes jointly responsible for local distortions and elastic anomalies are similar for both oxygen deficient and Gd-doped ceria.
Elastic dipoles in Ce0.8Gd0.2O1.9 
The analysis discussed herein above indicates that the cerium-oxygen vacancy, CeCe—VO, interaction forms an elastic dipole, which can easily reorient under external stress similar to Gorky or Snoek effects. However, the Gorsky and Snoek effects are usually observed in electrically conductive materials (metals or alloys). The uniqueness of the CeCe—VO elastic dipole is that at room temperature, Ce0.8Gd0.2O1.9 is a poorly conductive material. Therefore, CeCe—VO dipoles may reorient under an external electric field. The reorientation takes place by moving of the oxygen vacancy to the neighboring oxygen site. Since, Ce0.8Gd0.2O1.9 is a good ionic conductor, even at room temperature, the oxygen diffusion coefficient is ≈10−17 cm2/sec, which implies that the characteristic time necessary for the vacancy to shift to a neighboring site is about a minute. Therefore, application of the external electric field to Ce0.8Gd0.2O1.9 may result in a rearrangement of the elastic dipoles in the course of a few minutes. The effect is expected to be two fold: a) an applied electric field may result in strain and stress directly and b) application of the external field may affect the “effective” elastic modulus of Ce0.8Gd0.2O1.9 by suppressing the chemical strain effect. Therefore, if the material is under externally imposed strain (stress), then suppression of the chemical strain effect by an external electric field should manifest itself as an “increase” of the effective elastic modulus.